Coupling circuit



/A/VEN TOR s. 006,4, Jn. Br

AT oR/VEV S. DOBA, JR

COUPLING CIRCUIT Filed Nov. 10, 1939 Dec. 24, 1940.

l FREQUENCY-KC.

Patented Dec. 24, 1940 UNITED STATES PATENT oFFlCI 2,226,238 coUPLING CIRCUITl Stephen Doha,

Jr., Long Island City, N. Y., as-

signor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 10, 1939, Serial No. 303,778

4 Claims.

an open wire or a cable telephone pair balanced k With'respect to such fixed potential, a suitable coupling circuit or network is required. At low frequencies or over a very narrow band of frequencies, a transformer or other coil arrangement may be satisfactory. At very -high frequencies, however, or over a wide band f frequencies, for example, a few cycles to several millions of cycles, as used in television, such expedients are not satisfactory.

Coupling circuits or networks not employing transformers or coils for connecting balanced and unbalanced lines are known, of course. They have not solved, however, the problem of eliminating the 'effect of longitudinal currents, nor

that of parasitic capacities that becomeimpo-rtant when the frequencies involved become of a very high order, for example, one or more megacycles.

object of this invention is to provide an improved circuit or network for coupling together balanced and unbalanced lines or net* works, wherein the effect of longitudinal currents is suppressed to a great extent. This suppression is relatively unaffected by variationsof the active elements (tubes) in the network caused either by changes in the ytubes themselves or by the effect of high voltages due to longitudinal currents.

A further object of this invention is to minimize the effect of parasitic capacities on the suppression of longitudinal currents of high frequencies.

The method whereby longitudinal currents may be suppressed is well known and consists of balancing these currents against each other so as to produce no effect on the unbalanced circuit at which the balanced circuit is connected. This method, however, does not produce a high order of suppression in a practical application for two reasons. These are: First, the active elements, such as vacuum tubes normally used, may vary from their proper adjustment and hence fail to provide a suitable balance. Secondythe longitudinal currents themselves may :be of 'such high values as to cause an unbalance due to non.

linearity of these active elements.

(Ci. 17a-#44) vThe present invention seeks to alleviate both these difficulties by means of negative feedback' in the active elements, and to compensate for the effect of-parasitic capacities by provision of a corrective or compensatory network in the in put circuit to one tube.

One embodiment in accordance with the invention comprises ak coupling circuit or network including a pair of electron discharge devices connected in push-pull relation and having an :unbypassed common `cathode impedance for providingnegative feedback to longitudinal currents effective at the input terminals ofthe coupling circuit connected, for example, to the terminals of" thel balancedrline 'or network. Any tendency `for 'the magnitude ofv feedback to diminish with increase inV frequency because of parasitic capacity across the feedback impedance, or between the input electrodes of the tubes, may be compensated for or neutralized by introducing a compensating network in the input circuit to one of the discharge devices.

A more complete understanding of this invention will be obtained from the detailed description which follows, read in conjunction with the appended drawing, wherein:

Fig. 1 is a simplified showing of the coupling circuit of' the invention;

Fig. 2 shows a circuit embodying a balancing amplifier vcircuit incorporating the invention;

Fig. 3 shows curves evidencing the longitudinal currents suppressioncharacteristics of the circuitof Fig. 2; and' Fig. 4fshows av network in accordance with this invention for insertion in the input circuit of one tube of the circuit of Fig. 1.

In the transmission of television signals over a telephone cable or open wire pair, one of the problems involved lies in the high noise currents encountered, resulting principally from 60-cycle power interference. Because of the wide frequency band to be transmitted and the rigid requirements on phase shift and loss of the signal, repeatingco'ils which would suppress these noise currentsare not now available. The balancing amplifierto be described discriminates between longitudinal and metallic circuitcurrents, with discrimination of a high order being obtainable at low frequencies and particularly in the range of power frequencies". This lo-ss to longitudinal currents falls off-at higher frequencies, but, in a manner also to be described, may be extended to very high frequencies.

The simplified showing of Fig. 1 will facilitate understanding of the invention. The circuit represents two electron discharge devices V1, V2 arranged in push-pull relation, adapted to be connected at terminals I, 2 to the terminals of a line balanced with respect to a iixed potential, speciiically, ground, and adapted to be connected at terminal 3 to the ungrounded terminal of a line unbalanced with respect to ground. The tubes may be multigrid devices, specifically pentodes, but inasmuch as the plate current of a pentode is a denite fraction of the total cathode current in any operating condition, for clarity and ease in developing the conditions for suppression of longitudinal currents, V1 and V2 are shown as triodes. An unbypassed resistance R is common .to the grid-cathode and cathode-anode circuits of the push-pull stage. The input voltages to the balancing amplier are designated e1 and e2, and the respective currents of V1 and V2 are designated i1 and i2. Co, C1, C2 designate parasitio capacities that become effective only at high frequencies. The operation of the circuit may be explained as follows: When the input voltages are equal and of opposite polarity to ground, the condition for a signal or metallic circuit input, there is no appreciable potential drop appearing across the resistance R. The output circuit current i1 is then given by Z.1 1161 where g1 is the mutual conductance cf V1.

When, however, the input is due to a longitudinal current, e1 and e2 are of the same polarity from ground, and the potential drop across the resistance R is out of phase with e1. If the effect of the capacitances Co, C1, C2 is neglected, and where g1, ,L1 and g2, b2 refer to the mutual conductances and amplification factors of V1 and V2,

respectively, it can be shown that when e1=e2, and where the approximation is valid if gigz, ,Lui/r2, Rg2 l. The current i1 can be made Zero when where (in R92 is the logarithmic dinerential, or the percentage variation in gain of the tube V2. The physical meaning of Equation 4 is that the relative output due to longitudinal inputs is proportional to the per cent variation in the gain from the perfect balance point, and inversely proportional to the factor 2R91. The latter is the feedback stabilizing effect of the resistance R in the common feedback lead.

Fig, 2 shows a coupling circuit actually constructed in accordance with this invention. The balanced line HJ comprised a pair of conductors in a telephone cable, and the unbalanced line Il comprised a length of coaxial cable with the outer conductor grounded. The coupling circuit I2 comprised the pentodes V1 and V2. The resistances r, r are the terminating resistances for the balanced line. R is the common cathode resistance and corresponds to R of Fig. 1. The balance requirement as indicated by Equation 2 is obtained by means of the dividing network comprising the resistances R2 and R3 at the input of V1. The output circuit of the tubes comprises a network constituted by the resistance R4, capacity C4 and inductance L1. Plate power is provided by battery B. The screen grids of the tubes are connected together and to the positive side of plate battery, and also to a resistance R5 connected in shunt of the battery. The potential drop across R is quite large, and resistance R5 provides a means of balancing or bucking-out as much of this voltage as is desired. 'Ihe potentiometer tap P permits an adjustable control of the b-ias on the tubes V1 and V2. The balance setting for the push-pull stage is most readily obtained by variation of the grid bias, by means of the potentiometer, on the control grids of V1' and V2 simultaneously, Equation 3 being independent of g1. The resistance of the dividing network may be made as high as desired so that the resistances r may be varied between wide limits to suit different line termination impedances. Condensers C6 and resistances Re are the usual blocking condensers and grid leak resistances, respectively.

Curve A of Fig. 3 shows the suppression in decibels of the longitudinal currents with respect to frequency using the circuit of Fig. 2. This indicates that up to about l0 kilocycles the suppression was of a very high order, and that even up to one megacycle the suppression of longitudinal currents remained of very substantial magnitude.

The use of Equation 2 is valid only for low frela? b w (5) e1 .iw atm/g) MMWR) For balance, i120 when i Q (Hin) (1+jwC/g) (6) @2 A circuit for obtaining the condition indicated by Equation 6 would comprise a resistance connected between terminal l and the grid of V1 of Fig. 1; a second resistance connected from the grid terminal of the rst resistance to the ground side of resistance R.; and a capacitance and a resistance series-connected in shunt of the second resistance. In Fig. 2, R2 corresponds to the first resistance, and R3 to the second resistance. The series-connected capacitance and resistance would be connected in shunt of Rs to provide the complete corrective network. This network with the value of its elements expressed relative to that of resistances R1 and R to which resistances R3 and R correspond is shown in Fig. 4.

The dotted line B of Fig. 3 shows that theoretically the suppression of unbalance due to longitudinal currents will be substantially constant with frequency when such a corrective network is used. The broken line C, based on measurements made when a condenser and a resistance connected in series were connected in shunt with R3 of Fig. 2, indicates that a close approximation of the theoretical possibility may be obtained.

Although this invention has been disclosed with reference to a specic embodiment, it is evident that it is not limited thereto, but is of a scope determined by the appended claims.

What is claimed is:

1. A transmission system comprising a transmission line balanced with reference to a fixed potential, a second transmission line unbalanced with reference to said potential, and means coupling said transmission lines, said means com` prising a pair of electron discharge devices haw ing a plurality of elements, an impedance common to both of said devices and providing degenerative feedback for the suppression of longitudinal currents present in said rst line, and a network in the input circuit of one of said de- 40 vices to compensate for parasitic capacity effects whereby the suppression of longitudinal currents may be further increased.

2. A wide frequency band transmission system comprising a transmission line balanced with ref- 45 erence to a fixed potential, a second transmission line unbalanced with reference to said potential, and means coupling said transmission lines,` said means comprising a pair of electron discharge devices each having a cathode, an anode and a control grid, an impedance common to the cathodeanode and cathode-grid circuits of both of said devices and providing degenerative feedback for the suppression o longitudinal currents present in said first line, said suppression occurring to a preassigned degree upto a frequency at which parasitic capacities between grids and cathodes andthat of said impedance become important, and a corrective network in the input circuit of one'of said devices to compensate for the effect of said capacities whereby the same order of suppression exists to a substantially higher frequen- Cil put terminals, and means connected in the input circuit to one of said devices for compensating for the feedback reducing effect with rising input frequency of the distributed capacity of said common resistance and the interelectrode capacity between the grids and cathodes of said devices.

4. A coupling circuit. as claimed in the next preceding claim in which said means comprises a network comprising a resistance connected between one input terminal and the grid of said one device, a second resistance connected at one terminal to said rst resistance and at its other terminal to the end of said feedback resistance.

remote from said cathodes, and a series-connected capacitance and a resistance connected in shunt of said second resistance.

' STEPHEN DOBA, J R. 

